In bistatic and multistatic active sonars, used e.g., for tactical and surveillance applications, the transmitter is located at an appreciable distance from the receiver. The distance between the source and the receiver is generally much less than the length of the sound path from the source to the target to the receiver. The arrival of the transmitted waveform at the receiver directly from the transmitter, known as the "direct blast," is therefore much stronger than the target echo. Typical geometries show the direct blast to be 40 to 60 dB above typical target echoes. The direct blast can arrive simultaneously with echoes of previously transmitted waveforms from targets at ranges of interest, preventing their detection.
Most spatial processing applications deal with interferences that are continuous (rather than pulsed like active waveforms), and much lower in level. A "spatial transient," occurring when the waveform leading or trailing edge is propagating across the array, can result in signal masking even when a spatial null is steered in the direction of the source. This transient is many dB below the overall direct blast level, but may still mask the signal because the direct blast level is 40-60 dB above the signal.
Existing systems detect the presence of the direct blast by using desensitized hydrophones in the receiving array to determine when the direct blast is present. These systems use this information to "blank" system operation during the entire direct blast, either by reducing gain into the system during the entire direct blast period or by simply not processing data. Blanking prevents detection of signals masked by the direct blast, even though they arrive from different directions than the source direction.
Spatial processing techniques, either through design of low sidelobes, null steering, or adaptive methods, attempt to reduce the level of the direct blast when it arrives from a different angle than the target of interest. When the direct blast is 40 to 60 dB above the signal, the spatial transient is still strong enough to limit detection of many signals. The spatial transient is a unique phenomenon related to the direct blast in multistatic systems (or other transient interferences many orders of magnitude larger than the signal of interest). Conventional spatial rejection techniques, whether adaptive or fixed parameter approaches, will exhibit the spatial transient and will not, therefore, effectively reject these strong interferences to a degree which allows detection of a signal 40-60 dB lower in level.
The effects of direct blast can be reduced by several techniques. The sonar can be operated in range-doppler bins other than that containing the direct blast when the signal to be detected is an echo of the same waveform as the direct blast. While it cannot be guaranteed that the target will appear at a different doppler than the direct blast, the ability to operate in other range bins greatly reduces the deleterious effect of the direct blast. The rejection of the direct blast in range-doppler bins sufficiently far from the blast in frequency can be on the order of 36 dB for waveforms typically used in these systems.
Another technique for reducing direct blast interference is to use a series of waveforms with low cross-ambiguity, so that a direct blast due to one waveform is reduced by the matched filter used to detect the echo of another waveform. However, many long range systems use only Pulsed Continuous Wave (PCW) and Hyperbolic Frequency Modulated (HFM) waveforms due to computational considerations. This results in a limited selection of low cross ambiguity waveforms. The cross-ambiguity of typical low frequency sonar waveforms is on the order of 20-25 dB, but can be as low as 10 dB.
Transmitting successive waveforms in separate receiver sub bands with very good out of band rejection, say 60-80 dB, can also be used to reduce direct blast interference. If the direct blast and the received echo fall in different sub bands, the direct blast level is reduced by the out of band rejection relative to the signal. Practical considerations limit the number of sub bands in a given system to a relatively small number.
Spatial rejection reduces the direct blast level provided the source and target arrive from different directions. This is implemented in a beamformer, which achieves a directional response in the direction of the signal by computing a weighted sum of the delayed (or weighted) hydrophone outputs, with the delays and weights selected to yield the desired spatial response. Conventional beamformers maximize the response in the signal direction while reducing the response in all other directions to less than the sidelobe level. Practical systems achieve sidelobe levels of 20-25 dB.
For a direct blast that is 60 dB above the signal, a positive signal-to-noise ratio is typically achieved only when the echo and direct blast are in different sub bands. Thus, additional rejection is required if the target is to be detectable in other cases.
In passive systems, rejection of interferences is often achieved by steering of spatial nulls in the direction of the interference if the location of the interference is known with sufficient accuracy, or by means of various adaptive techniques which effectively steer a null if the interference is strong enough. Similar techniques have been used in active sonars to reject strong, continuous interferences or interferences whose duration is long in comparison to that of the transmitted waveform. In the case of the direct blast, however, the interference duration is nominally the same as that of the echo, and the direct blast-to-noise ratio is much higher than interference-to-noise ratios generally encountered. The combination of these differences results in an effect in the spatial processing that differs from cases usually considered. However, while the very high interference-to-noise ratio associated with the direct blast requires some changes to conventional spatial processing techniques, it does allow accurate determination of the location of the arrival direction of the direct blast.